What should you do if you see glow-in-the-dark clouds high in the evening sky? Report them, because atmospheric scientists are startled to see them in the skies of Colorado and Utah.

"It’s so unusual, it’s like seeing a polar bear walk down the Pearl Street Mall," says Gary Thomas, an expert on glow-in-the-dark, or noctilucent clouds (NLC), at the University of Colorado at Boulder’s Laboratory for Atmospheric and Space Physics.

To those who have seen them, NLC are truly a wonder. "They are ice-crystals that are shining in the sunlight above the earth’s shadow... and appear to be almost luminescent against the dark sky background," Thomas explains.

To scientists studying the phenomenon, however, noctilucent clouds are not just a nocturnal light show. Many believe the glow-in-the-dark clouds are a signature of global atmospheric changes caused by pollutants, as people spot them farther away from their former neighborhood in the Arctic regions.

On the March?

The term noctilucent is derived from the Latin word nocti, for night, and lucent, for luminous. NLC are the highest clouds on earth, forming 50 miles overhead, in the region of the upper atmosphere known as the mesosphere. At 135 Kelvin—approximately 160 degrees below zero—the mesosphere is the coldest area of the planet. So far, NLC have only been seen in the summer months, when the upper atmosphere is at its coldest.

Although they are visible to the naked eye, no one had seen a noctilucent cloud until a hundred years ago. Thomas believes it is the rise of manmade pollution that brought them to populated areas. He
says greenhouse warming in the lower atmosphere forces moisture and abnormally cold air into the upper atmosphere. "Our idea is that these are actually artificial in that they were created by the emissions from mankind’s production of methane and carbon dioxide," Thomas says.

Until recently, NLC were only seen in latitudes above 50 degrees North—that is, north of the United States/Canada border. They have also been spotted in Antarctica, although the best viewing locations in the Southern Hemisphere are in the ocean.

Ten years ago, Thomas predicted the clouds would appear more often and outside the polar region due to global warming. But that wasn’t supposed to happen until the late 21st century.

"I don’t think anyone would have dreamed these changes would have been that profound," Thomas says. "They appear to be on the march."

Exciting Sightings

Mike Taylor couldn’t believe his eyes when he glanced up at the sky the evening of June 22, 1999, from his backyard in Logan, Utah. Taylor, an atmospheric scientist, used to fly halfway around the world to study noctilucent clouds. He realized immediately that his sighting at 42 degrees North latitude was a record-breaker. "I was astounded and, at the time, extremely happy," says the Utah State University researcher. "I was running around trying to get my wife to find a camera with film in it."

Taylor not only documented his observations in home video and photographs, he also called his colleague, physicist Vincent Wickwar. Sure enough, Wickwar’s LIDAR beam confirmed the clouds were true NLC. The clouds appeared again over Utah the next night, and Taylor and Wickwar gathered more evidence.

Taylor alerted the Salt Lake Tribune. Records were smashed again a few days later by an amateur sighting near the Colorado/New Mexico border!

Taylor is convinced the sightings are not a fluke, but a trend. "I personally believe we really are seeing a change in the upper atmosphere, and Mother Nature is emphasizing this by letting us see them at lower and lower latitudes," he says. He now keeps two cameras mounted on tripods facing north, awaiting the clouds’ next appearance.

Canaries of the "Ignorosphere"

Noctilucent clouds may be a sensitive indicator of global climate changes—Thomas calls them "the miner’s canary" of the mesosphere—which is traditionally poorly understood. "It used to be called the ’ignorosphere,’" he laughs.

The ice clouds’ advance will likely also advance atmospheric studies. Researchers at the University of Michigan took mesosphere temperatures during the recent sightings aboard NASA’s Upper Atmosphere Research Satellite.

Because NLC are so easily viewed, Thomas says amateurs are important contributors to scientific research, and hope to encourage everyone to report any new sightings. You may even make it into the record books!

You Can Help Track Noctilucent Clouds

The season for spotting NLC in the Northern Hemisphere ranges from May to mid-August. Thomas says the clouds could potentially show up anywhere, but if you live in Florida, there’s still little chance of seeing the show.

To be an NLC observer:

• Scan the northern sky an hour after sunset, or one hour before sunrise, looking in the direction of the sunset or sunrise.
• NLC appear low on the horizon, shining a pearly white against the dark twilight sky, often highlighted by rippling bluish waves.
• If the moon is up, NLC may be confused with moonlit cirrus clouds. For a simple test, look at the clouds through a polarized pair of sunglasses. Rotate the lens 90 degrees; if the cloud changes its brightness, it is polarizing the light, a good signature of NLC.
• Take a picture using a tripod with a 10- to 30-second exposure. Thomas recommends 200 ASA color film.
• Record the date, time and your location.
• Report your observations by email to NLC CAN AM, a North American NLC network founded by amateur observer Mark Zalcik.

Elsewhere on the Web

Noctilucent Cloud Observers’ Homepage

Noctilucent Cloud Page

Observing Noctilucent Clouds

NOAA Aeronomy Laboratory